This invention relates to the chemical arts. In particular it relates to a method for the preparation of a crosslinked polymer of dicyclopentadiene (hereinafter referred to as DCPD) with a metathesis-catalyst system. A homogeneous binuclear catalyst which incorporates a known metathesis catalyst, such as WOCl.sub.4, and a complex of a different transition metal halide, which by itself may be inactive for metathesis, is used. The incorporation of the second metal complex improves the activity of the catalyst. The binuclear catalyst is activated by traditional metathesis catalyst activators, such as alkyl aluminum compounds.
Preparation of thermoset cycloolefin polymers via metathesis catalysts is a relatively recent development in the polymer art. Klosiewicz, in U.S. Pat. Nos. 4,400,340 and 4,520,181, teaches preparation of such polymers from dicyclopentadiene and other similar cycloolefins via a two-stream reaction injection molding technique wherein a first stream, including the catalyst, and a second stream, including a catalyst activator, are combined in a mix head and immediately injected into a mold where, within a matter of seconds, polymerization and molding to a permanently fixed shape take place simultaneously. These polymers have high impact strength and high modulus.
In the presence of a metathesis catalyst system, polymerization takes place extremely rapidly even at low temperatures. Polymerization occurs so rapidly that it is not unusual for the monomer to polymerize to a solid, immobile condition before the mixed streams can be transferred to the mold. To overcome this difficulty, Klosiewicz teaches the inclusion of a reaction rate moderator in the activator stream to delay the catalyst activation until the reaction mass is totally within the mold. The total time from mixing until polymerization is substantially complete is still just a matter of seconds.
In the typical system, according to Klosiewicz, the catalyst component is a tungsten or molybdenum halide and the activator is an alkyl aluminum compound. The reaction rate moderator can be an ester, ether, ketone or nitrile.
A reaction injection molding process involves the mixing of two or more low viscosity reactive streams. The combined streams are then injected into a mold where they quickly set up into a solid infusible mass. For a RIM system to be of use with a particular polymer, certain requirements must be met: (1) the individual streams must be stable and must have a reasonable shelf-life under ambient conditions; (2) it must be possible to mix the streams thoroughly without their setting up in the mixing head; (3) when injected into the mold, the materials must set up to a solid system rapidly; and (4) any additives-fillers, stabilizers, pigments, etc. must be added before the material sets up. Therefore, the additives selected must not interfere with the polymerization reaction. The catalyst should also have sufficient activity that the polymerization is essentially complete, that is, so that the residual monomer is minimized. The presence of residual monomer may have a deleterious effect on properties, such as stiffness, and may impart an objectionable odor to the part.
It is the object of this invention to provide cycloolefin polymerization catalyst systems having substantially increased activity compared to catalyst systems previously employed. It is further an object of this invention to promote more complete polymerization of the dicyclopentadiene monomer.
Homogeneous olefin metathesis polymerization catalysts are commonly based on WCl.sub.6, activated with alkylating agents such as alkyl aluminum or alkyl tin compounds. Studies have shown that it is actually WOCl.sub.4, which forms by contact with moisture, which is the major contributor to activity of catalysts based on WCl.sub.6. Pure WOCl.sub.4 is also an effective metathesis catalyst. Recently, Martin in U.S. Pat. No. 4,696,985 disclosed that a mixture of WCl.sub.6 and WOCl.sub.4 activated with an aluminum alkyl or an aluminum alkyl halide, had greater activity for metathesis of strained cyclic olefins than either component alone. The controlled addition of an oxygen donor to WCl.sub.6 to form a mixture of WCl.sub.6 and WOCl.sub.4 improved catalyst activity.
In U.S. Pat. No. 4,703,068 a tantalum pentachloride catalyst was disclosed as being employed in a method of making a cross-linked substantially transparent low haze polymer. Other transition metal complexes can also act as homogenous metathesis catalysts, although their activity tends to be lower than those based on tungsten or molybdenum. There are only a few examples of two different transition metal complexes being used together as metathesis catalysts. In most cases, these are heterogeneous catalysts, which are not soluble in the monomer. Heterogeneous catalysts tend to be unsuitable for bulk polymerization because the catalyst becomes encapsulated in the polymerizing mixture and is not mobile, leading to incomplete polymerization. In other cases, the second transition metal complex serves as the activator component of a homogeneous metathesis catalyst, rather than as a component of the catalyst itself, which requires a third component as an activator.
Preparation of a thermoset polyDCPD by reactive liquid processing has been disclosed by Klosiewicz in U.S. Pat. No. 4,400,340. Typically, dicyclopentadiene monomer is used to prepare two or more reactive liquid streams. One stream contains the monomer and the metathesis catalyst activator ("A Component"), which is typically an aluminum alkyl or aluminum alkyl halide, or other metathesis activator. The A Component also typically contains the rate moderator. The second stream ("B Component") contains the monomer, other desired additives, and a homogeneous metathesis catalyst, or, more properly a catalyst precursor. This precursor is solubilized and stabilized to prevent any premature polymerization. Thus, two stable streams are obtained which can be processed in typical reactive liquid processing methods, such as RIM. Other materials, such as an elastomer, may be incorporated into either stream or into both streams. The elastomer imparts viscosity, which improves the processability and may also alter the polymer mechanical properties.
The catalyst solution typically used in this process is a homogeneous catalyst based on WCl.sub.6. The solution is prepared by dissolving WCl.sub.6 in an inert solvent, usually an aromatic solvent such as toluene or chlorobenzene. Activity is improved by controlled addition of an oxygen donor to produce a mixture of WCl.sub.6 and WOCl.sub.4. Next, the catalyst is solubilized by addition of a phenolic compound, such as an alkyl phenol. This intermediate solution has excellent metathesis catalyst activity. However, it is not optimum in a RIM process because it does not provide a stable solution in monomer, i.e. it has a short shelf life because it causes a slow polymerization of the DCPD. Thus, a stabilizing agent, typically a Lewis base, and preferably a chelating agent, is added. Most commonly, this is a material such as acetylacetone (2,4-pentanedione, abbreviated as acac). Finally, this catalyst solution is added to the monomer stream to form the B Component. When the two streams are mixed, exothermic polymerization takes place within a short time, frequently less than one minute. Thus, useful articles can be fabricated directly from the monomer, reducing the cost of polymer processing.
It has been found desirable to find means to retain and to increase the activity of the catalyst when using the moderators and stabilizing agents. This invention discloses a means of increasing the activity of the catalyst by introducing a second component which is a complex of a transition metal other than the tungsten of the catalyst used in the prior art.